This invention relates to a process for producing a gasoline of reduced sulfur content without a loss in yield, while olefin saturation is minimized to preserve octane-barrel value. More specifically, the invention relates to low temperature process for producing a gasoline of reduced sulfur content which advantageously relies on either a dual functional catalyst, such as CoMo ZSM-5/Al.sub.2 O.sub.3, or a conventional catalyst, such as CoMo/Al.sub.2 O.sub.3, to maximize the octane-barrel value of the product gasoline.
Catalytically cracked gasoline currently forms a major part of the gasoline product pool in the United States and the cracking process contributes a large proportion of the sulfur in the gasoline. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Low sulfur levels result in reduced emissions of CO, NO.sub.x, and hydrocarbons. In addition, other environmental controls may be expected to impose increasingly stringent limits on gasoline composition. Currently, the requirements of the U.S. Clean Air Act and the physical and compositional limitations imposed by the Reformulated Gasoline ("RFG") and EPA Complex Model regulations will result not only in a decrease in permissible sulfur levels but also in limitations on boiling range, typically measured by minimum Reid Vapor Pressure ("RVP") and T.sub.90 specifications. Limitations on aromatic content may also arise from the Complex Model regulations.
Cracked naphtha, as it comes from the catalytic cracker and without any further treatments, such as purifying operations, has a relatively high octane number as a result of the presence of olefinic components. In some cases, this fraction may contribute as much as up to half the gasoline in the refinery pool, together with a significant contribution to product octane. Other unsaturated fractions boiling in the gasoline boiling range, which are produced in some refineries or petrochemical plants, include pyrolysis gasoline and coker naphtha. Pyrolysis gasoline is a fraction which is often produced as a by-product in the cracking of petroleum fractions to produce light unsaturates, such as ethylene and propylene. Pyrolysis gasoline has a very high octane number but is quite unstable in the absence of hydrotreating because, in addition to the desirable olefins boiling in the gasoline boiling range, it also contains a substantial proportion of diolefins, which tend to form gums after storage or standing. Coker naphtha is similar in containing significant amounts of sulfur and nitrogen as well as diolefins which make it unstable on storage.
Hydrotreating of any of the sulfur containing fractions, which boil in the gasoline boiling range, causes a reduction in the olefin content and consequently a reduction in the octane number. As the degree of desulfurization increases, the octane number of the normally liquid gasoline boiling range product decreases. Some of the hydrogen can also cause some hydrocracking as well as olefin saturation, depending on the conditions of the hydrotreating operation.
Naphthas and other light fractions such as heavy cracked gasoline can be hydrotreated by passing the feed over a hydrotreating catalyst at an elevated temperature and a somewhat elevated pressure in a hydrogen atmosphere. One suitable family of catalysts which has been widely used for this service is a combination of a Group VIII metal and a Group VI metal of the Periodic Table, such as cobalt and molybdenum, on a substrate such as alumina. After the hydrotreating operation is complete, the product can be fractionated, or simply flashed, to release the hydrogen sulfide and collect the now sweetened gasoline.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. The sulfur impurities tend to concentrate in the heavy fraction of the gasoline and hydrodesulfurization processes have been employed that treat only the heavy fraction of the catalytically cracked gasoline so as to retain the octane contribution from the olefins which are found mainly in the lighter fraction. In one commercial operation, the selectivity for hydrodesulfurization relative to olefin saturation is shifted by suitable catalyst selection, for example, by the use of a magnesium oxide support instead of the more conventional alumina.
In any case, regardless of the mechanism by which it happens, the decrease in octane which takes place as a consequence of sulfur removal by hydrotreating creates a conflict between the growing need to produce gasoline fuels with higher octane number and--because of current ecological considerations--the need to produce cleaner burning, less polluting fuels, especially low sulfur fuels. This inherent conflict is yet more marked in the current supply situation for low sulfur, sweet crudes.
Aromatics are generally the source of high octane number, particularly very high research octane numbers, and are, therefore, desirable components of the gasoline pool. However, they have been the subject of severe limitations as a gasoline component because of possible adverse effects on the ecology, particularly with reference to benzene. Thus, it has become desirable, as far as is feasible, to create a gasoline pool in which the higher octanes are contributed by the olefinic and branched chain paraffinic components, rather than the aromatic components.